<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>三角形</title>
    <style>
        .angle{
            width: 0;
            height:0;
            border: 20px solid #1a095b;
            border-left:20px solid transparent;
            border-right:20px solid transparent;
            border-bottom:20px solid transparent;
            margin: 100px auto;
            transition: all 0.3s;
        }
        .angle:hover{
            transform: rotate(180deg) translateY(10px);
        }
    </style>
</head>
<body>
    <div class="angle">

    </div>
    三角形的制作步骤：<ol type="1">
        <li>首先将width和height设置为0</li>
        <li>看三角形的箭头在哪边，箭头向下设置上边border</li>
        <li>然后设置其他三边的borderborder-XXX：20px solid transparent;</li>
    </ol>
</body>
</html>